Fifty years ago a few outstanding physicists, including Leonid Keldysh and the Nobel prize recipient Walter Kohn, put forward a heretic paradigm of a strongly correlated insulator [1-3]: If a narrow-gap semiconductor (or a semimetal with slightly overlapping conduction and valence bands) failed to fully screen its intrinsic charge carriers, then excitons---electron-hole pairs bound together by Coulomb attraction---would spontaneously form. This would destabilize the ground state, leading to a reconstructed ‘excitonic insulator’ that would exhibit a distinctive broken symmetry, inherited by the exciton character, as well as peculiar collective modes of purely electronic origin. Intriguingly, the excitonic insulator, which shares similarities with the Bardeen-Cooper-Schrieffer superconducting ground state, could display unusual macroscopic quantum coherence effects [4-8]. So far, the observation of this phase has been elusive. The crux of the matter is the trade-off between competing effects in the semiconductor: as the size of the energy gap decreases, favouring spontaneous exciton generation, the screening of the electron-hole interaction increases, suppressing the exciton binding energy.
Very recently, novel low-dimensional systems and quantum devices seem to renew the promise of the excitonic insulator, as they combine optimal band structures, poor screening behavior, truly long-ranged interactions, and giant excitonic effects. These include systems as diverse as carbon nanotubes [9], low-dimensional [10] and van der Waals [11-14] heterostructures, Dirac and Weyl materials [15-17], topological insulators [18]. By collecting the key actors of theoretical and experimental research, who are spread among different communities, this Workshop aims at in-depth analysis of common themes and challenges, both theoretical and computational, to establish a roadmap to the excitonic insulator.

An additional list of recent works on the excitonic insulator is maintained at Cnr-Nano website